1. Field of the Invention
This invention is in the field of absorption spectroscopy, more particularly, it pertains to optoacoustic absorption spectroscopy of condensed matter in bulk form.
2. Description of the Prior Art
Conventional optical spectroscopy techniques tend to fall into two major categories, namely, one can study and measure either the optical photons that are transmitted through the material under study, or those that are scattered or reflected from that material. During the past several years, another optical spectroscopy technique has been developed. This technique, called "optoacoustic" (OA) spectroscopy, is distinguished from the conventional techniques chiefly by the fact that even though the incident energy is in the form of optical photons, the interaction of these photons with the material under investigation is studied not through subsequent detection and analysis of photons, but rather through measurement of the energy absorbed by the material due to its interaction with the incident beam. One of the main advantages of OA spectroscopy is that in principle it enables one to obtain spectra, similar to optical absorption spectra, of any type of solid, liquid, or gaseous material. A recent review of the field of OA spectroscopy can be found in the article by Allan Rosencwaig, "Photoacoustic Spectroscopy" in Advances in Electronics and Electron Physics, Volume 46, Academic Press, (1978). We will restrict our discussion to OA methods that use piezoelectric transducers to detect the elastic strain waves generated by the absorption of electromagnetic energy in the sample, and that are applicable to bulk samples of condensed matter, where by "bulk samples of condensed matter," we mean solid or liquid (including suspensions and the like) samples having all three dimensions or roughly comparable magnitude, and having a total volume of the order of approximately 0.1 cm.sup.3 or larger.
The principles of OA spectroscopy relevant to this application are the same whether the sample is in solid or liquid form. Typically, part of the sample volume is illuminated with an intermittent beam of essentially monochromatic electromagnetic radiation of frequency .nu.. This illuminated sample volume we will refer to as the "source region." The matter contained in the source region absorbs radiation from the beam in proportion to .alpha.(.nu.), the absorption coefficient for radiation of frequency .nu.. This energy appears as heat energy in the sample and causes at least the source region to expand, resulting in elastic strain in the sample. This strain can be detected for instance by appropriately placed piezoelectric transducers, and from the transducer output .alpha.(.nu.) can be determined.
Prior art methods of OA spectroscopy have used either chopped CW radiation, with a chopping frequency typically near 1 KHz, or pulsed laser radiation, typically involving pulses of nanosecond duration. These methods typically allowed determination of absorption coefficients larger than about 10.sup.-5 cm.sup.-1. For instance, A. Hordvik and H. Schlossberg, Applied Optics, Vol. 16, pp. 101-107 (1977), describe OA apparatus that allows measurement of .alpha.(.nu.).apprxeq.10.sup.-5 cm.sup.-1 using CW power of a few hundred mw, resulting in power input into the sample of the order of 10.sup.-1 watt/cm.sup.3 or larger, and employing chopping frequencies between 150 and 3000 Hz. On the other hand, A. M. Bonch-Bruevich et al, Optics and Spectroscopy, Vol. 42, pp. 45-48 (1977) describe OA measurements that use nanosecond laser pulses and have a sensitivity sufficient only to detect the absorption of 10.sup.-6 .about.joules/pulse, resulting in a power input into the sample of the order of 10.sup.-6 watt/cm.sup.3 or larger.
These prior art variants of OA spectroscopy do not utilize the full potential of the method. In the case of millisecond and longer pulses (i.e., chopped CW) the response typically is reduced due to effects of thermal diffusion, and, in any case, it can be shown that such long pulses result in reduced sensitivity of the measurement. On the other hand, it is also easy to show that nanosecond pulses are nonoptimal because they typically result in a reduced response due to destructive interferences at the transducer between signals from different parts of the source region.